Usually, in an engine driven by gasoline, liquefied petroleum gas or the like, the compressed air-fuel mixture is fired by electric sparks generated by an ignition system. The ignition system is either a contacting type (electric type) or non-contacting type (electronic type), and generates electric sparks using a spark plug in response to high pressure applied from an ignition coil, thereby firing the air-fuel mixture.
FIG. 7(a) of the accompanying drawings shows an external appearance of a spark plug 10, and FIG. 7(b) is a cross section of the spark plug installed in a cylinder head H of an engine with inner parts.
Referring to those figures, the spark plug 10 includes a center electrode 2 axially housed in a cylindrical insulator 1. The center electrode 2 has one end sticking out via a bottom opening of the insulator 1. High pressure is generated by the ignition coil between positive and negative electrodes of a battery (not shown), and is applied between the sticking-out end of the center electrode 2 and to one end of an external electrode 3.
A lower half of the insulator 1 is enclosed in a spark plug housing 4, of which lower peripheral surface is a male-screwed part 4b. The spark plug 10 is fitted into a cylinder head H of the engine via the male-screwed part 4b. The lower end of the center electrode 2 comes out via a bottom opening of the male-screwed part 4b. The external electrode 3 is attached around a lower periphery of the male-screwed part 4b, and has its end facing with the lower end of the center electrode 2. Discharges are caused in a spark gap G between the end of the external electrode 3 and the end of the center electrode 2. The spark gap G faces with a combustion chamber of the engine.
An upper end of the center electrode 2 is connected to a terminal nut 5 fixedly fitted in an upper opening of the insulator 1. A high pressure cord C extends from an ignition coil connected to the positive electrode of the battery, and is connected to the terminal nut 5 via a cap 6.
Electric charges emitted in the spark plug gap G return to the negative electrode 11 of the battery from the external electrode 3 via the spark plug housing 4, the cylinder head H of the engine, an engine body, a chassis of the vehicle where the engine body is mounted, and so on. The electric charges tend to sluggishly flow while they pass over a long range through a number of contacts, engine body, chassis and so on.
Further, sometimes the spark plug 10 is loosely attached in the cylinder head H (i.e. at the male-screwed part 4b of the spark plug housing 4), or there are poor conductions caused by aged components of the engine body. In such a case, electric charges tend to flow non-smoothly, which will worsen the discharge performance. The lower the discharge performance, the weaker firing force, and the lower fuel efficiency. Further, engine torque characteristics will be worsened.
In the former case, the spark plug 10 is vibrated each time sparks are generated in order to fire the air-fuel mixture, so that the connection becomes slack between the spark plug 10 and the cylinder head H. In the latter case, engine components are always exposed to intense heat, and will be inevitably aged.
The invention has been contemplated in order to overcome problems of the related art, and to provide a grounding structure which can remove factors causing reduced discharge performance, enable electric charges to be effectively returned to a negative electrode of the battery, and to improve the fuel efficiency and engine torque characteristics.